Our long-term research interest is to provide mechanistic links between cellular lipid metabolism and neuropathology. In a new manuscript (included in the Appendix) and in the PRELIMINARY STUDIES section, we describe the following leads: 1. We show that genetic inactivation of acyl-CoA: cholesterol acyltransferase 1 (ACAT1) increases 24(S)-hydroxycholesterol content, reduces cholesterol synthesis in the brain, and ameliorates amyloid pathology in the triple transgenic Alzheimer's mice (AD mice). 2. We show that the cholesterol content, the mRNA of SREBP2 (the transcription factor that controls genes involved in cholesterol biosynthesis), the mRNA of HMGR (the rate-limiting enzyme in cholesterol biosynthesis), and the mRNAs of two interacting transcription factors, serum response factor (SRF) and myocardin (MYOCD) are elevated in the AD mice compared to nontransgenic mice. Based on these leads, in the current proposal, we design experiments to test two hypotheses. The first is to test whether inactivating the gene that encodes the cholesterol esterification enzyme ACAT1 in the brain has therapeutic value for treating AD. The second is to test if specific sphingolipid(s) mediate the action of amyloid beta peptide 1-42 (Abeta1-42) by stimulating cholesterol biosynthesis in the brain. We enlist three specific aims: Specific Aim 1: To test the effect of inactivating the enzyme 24(S)-hydroxylase CYP46A1, on Acat1-/- (A1-) mediated modulations on hAPP, HMGR and ABCA1 in AD mice neurons. Specific Aim 2: To test the effect of inactivating Acat1 in AD mouse brains at different time during life. Specific Aim 3: To study the effects of Abeta1-42 on cholesterol and sphingolipid metabolism in primary neurons and astrocytes.